The invention relates to a piston and cylinder unit in the form of a hollow cylindrical member with a circular cross section, and whose ends are shut off by end caps. Inside the cylindrical member there is a piston with a circular cross section running axially so as to make sealing contact with the bore face of the cylindrical member. A piston rod is secured to the piston and extends from it out of the cylindrical member through one of the cylindrical member end caps so that the rod is able to be extended from said cylindrical member and retracted back into it. Furthermore there are means to prevent relative twisting between the piston rod and the cylindrical member.
In piston and cylinder units of this type the means for precluding relative rotation between the piston rod and the cylindrical member usually includes a complex arrangement with a rod fixed and parallel to the piston rod so as to extend through a hole in a part mounted on one of the end caps. Such means for preventing relative twisting are however very difficult to provide if the unit has to be made very compact in form as a consequence of the small amount of space available, since the anti-twist arrangement includes the part arranged perpendicularly to the direction of advance of the piston rod so that the arrangement is comparatively broad in construction. A further point is that the arrangement is not very robust so that when the piston rod is extended to a considerable degree and a substantial torsion force is acting on the piston rod, the degree of accuracy of the twist preventing means will leave to be desired, i. e. there will be an unacceptable degree of backlash. Accordingly, when an anti-twist system has to meet heavy loads the designer may have to rely on a piston or piston rod outline departing from the circular form. However such forms of piston and cylinder arrangements are disproportionally expensive owing to the complexity of manufacture and this serves to exclude them from economic application to many systems needing a twist-preventing means.